1. Field of the Invention
The present invention relates to an electro-luminescence display device, and more particularly to an electro-luminescence display device that is adaptive for preventing picture quality deterioration by operating a thin film transistor for an electro-luminescence cell drive at a non-saturation area to compensate a threshold voltage, and a driving method thereof.
2. Description of the Related Art
Recently, there have been highlighted various flat panel display devices reduced in weight and bulk that is capable of eliminating disadvantages of a cathode ray tube (CRT). Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an electro-luminescence (EL) display, etc.
The EL display in such display devices is a self-luminous device capable of light-emitting a phosphorous material by a re-combination of electrons with holes. The EL display device is generally classified into an inorganic EL device using the phosphorous material as an inorganic compound and an organic using it as an organic compound. Such an EL display device has many advantages of a low voltage driving, a self-luminescence, a thin-thickness, a wide viewing angle, a fast response speed and a high contrast, etc, such that it can be highlighted into a post-generation display device.
The organic EL device is usually comprised of an electron injection layer, an electron carrier layer, a light-emitting layer, a hole carrier layer and a hole injection layer that are disposed between a cathode and an anode. In such an organic EL device, when a pre-determined voltage is applied between an anode and a cathode, electrons produced from the cathode are moved, via the electron injection layer and the electron carrier layer, into the light-emitting layer while holes produced from the anode are moved, via the hole injection layer and the hole carrier layer, into the light-emitting layer. Thus, the electrons and the holes fed from the electron carrier layer and the hole carrier layer emit a light by their re-combination at the light-emitting layer.
An active matrix EL display device using such an organic EL device, as shown in FIG. 1, includes an EL panel 20 having pixel cells 28 inclusive of EL cells OLED arranged at areas defined by intersections between gate lines GL and data lines DL, a gate driver 22 to drive the gate lines GL of the EL panel 20, a data driver 24 to drive the data lines DL of the EL panel 20, and a gamma voltage generator 26 that supplies a plurality of gamma voltages VH to VL to the data driver 24.
The gate driver 22 supplies a scan pulse to the gate lines GL to sequentially drive the gate lines GL.
The gamma voltage generator 26 generates different gray level gamma voltages VH to VL between a gamma voltage VL of high gray level and a gamma voltage VH of low gray level by use of n numbers of resistors connected in series between a ground voltage source and a supply voltage source (not shown), to supply the generated voltage to the data driver 24.
The data driver 24 converts a digital data signal inputted from the outside into an analog data signal by use of the gamma voltage VH to VL from the gamma voltage generator 26. And the data driver 24 supplies the analog data signal to the data lines DL whenever the scan pulse is supplied.
Each of the pixels 28 receives a data signal from the data line DL when a scan pulse is applied to the gate line GL to generate a light corresponding to the data signal.
For this, each of the pixels 28, as shown in FIG. 2, includes an EL cell OLED connected between the supply voltage source VDD and the ground voltage source GND, and a cell driver 30 to drive the EL cell OLED.
The cell driver 30 includes a switching thin film transistor T1, of which a gate terminal is connected to the gate line GL, a source terminal is connected to the data line and a drain terminal is connected to a first node N1; a driving thin film transistor T2 of which a gate terminal is connected to the first node N1, a drain terminal is connected to the supply voltage source VDD and a source terminal is connected to an anode of the EL cell OLED; and a storage capacitor Cst connected between the supply voltage source VDD and the first node N1.
The switching thin film transistor T1 is turned on when a scan pulse is supplied to the gate line GL, thereby supplying the data signal of the data line DL to the first node N1. The data signal supplied to the first node N1 is charged in the storage capacitor Cst and supplied to the gate terminal of the driving thin film transistor T2. The driving thin film transistor T2 responds to the data signal supplied to the gate terminal to control the amount of current Id supplied from the supply voltage source VD through the EL cell OLED. And, even the switching thin film transistor T1 is turned off, the driving thin film transistor T2 remains at an on-state by the data signal charged at the storage capacitor Cst, thus it can control the current amount Id supplied from the supply voltage source VDD through the EL cell OLED till a data signal of next frame is supplied.
On the other hand, each of the switching thin film transistor T1 and the driving thin film transistor T2 of the cell driver 30 uses an amorphous silicon layer as a semiconductor layer. At this moment, the amorphous silicon layer has a disadvantage of it mobility is low. Accordingly, a study for a poly silicon thin film transistor has recently been studied for using a poly silicon layer of excellent mobility as a semiconductor layer. The poly silicon thin film transistor can be integrated together with the driving drive integrated circuit in a substrate, thus there is an advantage that the degree of integration and price competitiveness is good. However, the strain temperature of glass is as low as 600° C., thus a crystal growth technique using high temperature of above 600° C. cannot be used in forming the poly silicon layer. Because of this, in forming the poly silicon layer, Excimer Laser Annealing (ELA) is generally used that an amorphous silicon layer is formed at a low temperature of 100˜300, then the amorphous silicon layer is heat-melted with a pulse illumination by an excimer laser of wavelength 308 nm, and then the melt silicon layer is crystallized in a cooling process. The poly silicon layer can be formed without giving any thermal damage to the glass substrate by use of the ELA.
However, the excimer laser has a characteristic that its optical power is unstable and the strength of output is changed within the range of ±10%. Because of this, in the ELA, there is a problem that the size of crystal grain in the poly silicon layer is irregular, and its re-productivity is bad. Also, the excimer laser has low repetition frequency of 300Hz in pulse driving, thus there are problems that it is difficult to continuously form a crystal grain boundary, high carrier mobility might not be obtained, and a large area cannot be annealed at a high speed.
The size, the size uniformity, the number and location, and the direction of the crystal grain of the semiconductor layer formed in the ELA process have critical influence directly or indirectly on the characteristic of thin film transistor, e.g., threshold voltage Vth, sub-threshold slope, charge carrier mobility, leakage current, device stability. Accordingly, the characteristic of the thin film transistor formed on the EL panel 20 by the ELA process becomes different by lines which correspond to the illumination direction of the excimer laser because the optical power of the excimer laser is unstable and its output strength is changed within the range of ±10%.
On the other hand, the operating point Q of the driving thin film transistor T2 generally exists in a saturation region as in the characteristic graph of a transistor in FIG. 3. This is because a stable current Id can be supplied to the EL cell OLED even though the voltage Vds between the drain terminal and the source terminal of the driving thin film transistor T2 is changed. At this moment, the amount of change of the current Id flowing in the driving thin film transistor T2 in the saturation region is bigger than in the non-saturation region, for the deviation of the threshold voltage Vth of each of the driving thin film transistors T2. Accordingly, for the voltage Vgs between the same gate terminal and the source terminal of each of the driving thin film transistors T2, if the deviation of the threshold voltage Vth, as described above, is big, the change of the current Id flowing in the driving thin film transistor T2 becomes big.
Accordingly, the EL display device of the prior art expresses the gray level by the change of the data voltage, thus in case that the threshold voltage Vth of the driving thin film transistor is not uniform for each line of the EL panel 20, the amount of the current flowing in the EL cell OLED cannot be accurately controlled (in fact, the current amount decreases) for the same data voltage, thus there is a problem that a desired picture is not displayed because the brightness is not uniform.